1. Field of the Invention
Preparation of cyclic diones by catalytic cyclization of certain delta-keto carboxylic acid esters is well known. Of particular interest herein are process improvements to the vapor-phase cyclization of delta-keto carboxylic acid esters.
2. State of the Art
U.S. Pat. No. 4,028,417 to Muller et al. describes the liquid-phase, two-step preparation of cyclohexane-1,3-dione type compounds by a first step of cyclization of 4-oxocarboxylic acid alkyl esters in the presence of a strong base such as sodium methylate to provide a sodium salt of the cyclohexanedione, and a second step of acidification of the sodium salt to form the cyclohexanedione. The two-step preparation requires an expensive solvent, such as an amide, sulfoxide, sulfone, or glycol dialkyl ether, in which to carry out the cyclization and acidification steps. Moreover, the Muller '417 preparation generates significant quantities of effluent, the recovery and disposal of which is costly.
U.S. Pat. No. 3,932,511 to Schaafsma et al. describes the preparation of cyclohexane-1,3-dione type compounds by vapor-phase cyclization of a delta-keto carboxylic acid ester in a hydrogen-nitrogen carrier gas stream passed over an activated carbon bed. The ester starting material is typically present in the carrier gas in a mole ratio of about 13-to-one carrier gas-to-ester. Recovery of cyclohexane-1,3-dione product and unreacted starting ester from the gas stream requires cooling the gas stream to at least -20.degree. C.
German Offen. No. 2,825,170 of Muller et al. describes the preparation of cyclohexane-1,3-dione type compounds by vapor-phase cyclization of a delta-keto carboxylic acid ester over a catalyst provided by carbon, alumina, or like support, impregnated with salts of Group IIIB or IVB elements. The ester is carried in a vapor state in a gas stream provided by an equimolar mixture of nitrogen and an ethylene glycol ether. Since the cyclohexane-1,3-dione has high solubility in aliphatic ethers, such as ethyleneglycol ether, removal of cyclohexane-1,3-dione product and unreacted keto ester requires a distillation step. In addition to requiring a complex, expensive catalyst, the Muller '170 preparation involves a two-step process for removing cyclic dione product and unreacted delta-keto ester. Firstly, the gas stream must be cooled to condense the ether to a liquid in which the cyclic dione product and keto ester are dissolved; secondly, the liquid ether must be distilled from the cyclic dione and unreacted ester, and distillation must occur at a sufficiently low temperature to remove the ether without destroying the heat-sensitive cyclic dione.
There remains, therefore, a need for a simple catalytic vapor-phase cyclization process for making 1,3-cyclic diones from delta-keto esters requiring no expensive refrigeration or condensing equipment to separate the cyclic dione product from the vapor-phase carrier.